Ankylosing spondylitis (AS) is a painful, incurable autoimmune disease that affects millions of people worldwide. Although it is clear that both environmental and genetic factors are involved in the disease, the cause and pathogenesis of AS remain unclear. A strong link with the MHC class I allele HLA-B27 has implicated antigen processing as an underlying factor, and this hypothesis was strengthened by the recent demonstration that polymorphisms in ERAP1 (an aminopeptidase that we and others have shown is associated with antigen processing) affect the risk of developing AS. We hypothesize that ERAP1 polymorphisms either directly or indirectly affect substrate selection and trimming, so that different ERAP1 alleles alter the levels of arthritogenic or protective peptides presented on HLA-B27. We will test this hypothesis using biochemical assays for substrate specificity, cultured cells to measure effects on antigen processing, and transgenic mice to test effects on disease in vivo. We will also test the effects of ERAP1 polymorphisms on other pathways that have been proposed to affect AS pathogenesis, including the assembly and stability of HLA- B27 and shedding of cytokine receptors. The Aims of this project are to determine the functional effects of ERAP1 polymorphisms and to understand the molecular pathogenesis of AS. The long-term goals of this project are to predict the risk of AS in individuals, and to develop methods to prevent and treat AS. We will also apply the understanding of AS pathogenesis to other autoimmune diseases that are linked to MHC class I alleles.